OP, for both planes that you're on, I'd suggest row 21 (back exit row) as the best. I might avoid the bulkheads because (someone please correct me if I'm wrong) there are no cut-outs so it will be difficult to stretch your legs.

The specific E+ seat choice otherwise is mostly due to preference. Here are the Seatguru maps in case you haven't found them yet:
http://www.seatguru.com/airlines/Uni..._737-900_B.php
http://www.seatguru.com/airlines/Uni..._767-300_D.php